I used DEARMAN with a particularly difficult topic over the summer -- both with my SO and her OSO -- and it went very well. I got what I asked for out of it, and it really helped me to be less nervous about having the conversation in the first place, because I had practiced what I actually needed to say and didn't just launch into it in some long and rambling way.
I had been invited on camping trip by friends that was a revisiting of a place we'd camped two years prior. I enjoyed the trip then, but it was after my exH and I had split up, and it was our first time both of us camping with a group and being in separate tents. I remember I had to buy some new camping gear before it because we'd split ours up during the divorce.
So, this new camping trip came up, and I RSVPed yes. I invited my SO, and she said she'd like to go with me. Then, friends of ours began inviting other mutual friends, and before long it was becoming a trip where it would be kind of weird not to invite my SO's OSO also. She knew probably a dozen of the people going, though most of them were people she had met through me, and no one was specifically inviting her because I think they were waiting to see whether I would. I had the feeling that I wouldn't mind inviting her, but I didn't want to share my partner with her on such a trip. If she went, I'd like it to be in addition to me and my SO going together as a couple as already planned -- i.e., no awkwardness about us being together, being affectionate, no trying to balance the time equally and all of that BS. It was also complicated by the fact that she was planning her own trip to another state the following week, and I didn't even know whether she'd consider going or not.
So, I asked my SO's OSO to meet me for a drink after work and let her know there was something I was a little nervous about but wanted to discuss with her in terms of planning and whatnot. I used DEARMAN to get myself ready for the talk, and then I basically repeated my thoughts out loud to her.
Describe: I described how the invitation had come about, that the situation was making me feel awkward, that I felt it was something we should talk about, etc.
Express: I expressed that I had emotions related to this particular camping trip and camping site (e.g., that I'd had an awkward time there in the past because of my split with my exH and didn't want to have an awkward time again because of our complicated relationship situation; that I wanted to be able to invite my SO on group outings like any normal couple without feeling responsible for everyone else's feelings; that I had been specifically invited and had invited my SO early and didn't particularly want to change the feeling of this trip; that I might prefer not to go rather than go and have it be all kinds of awkward; I expressed that I actually thought the OSO might enjoy going on the trip, too, but that I hadn't been sure how to broach the topic, etc.).
Assert: I invited her to come if she thought she would be able to enjoy it but that I did not want to do any awkward time balancing/tent swapping/whatever. I asked for this to be me and my SO's camping trip together, regardless of whether the OSO thought she might want to go as well.
Reinforce: I let her know that I thought we all deserved to have this sort of thing -- that my SO and her would be going on their own trip the following month, that this would allow me and my SO to have a much needed opportunity to be a relaxed couple around our friends, that I understood they might have such things come up in the future themselves and would respect a request for it to be their private trip (they do occasionally get invited to stuff like a dinner party, and I don't feel it's their responsibility to automatically extend the invitation to me), etc.
Mindful: I kept my mind on this topic, rather than getting all worked up about whether I was discussing it in the best order or how unusual it was or what she might be thinking or whatever.
Appear confident: I didn't back down from my request. I considered it the adult thing to do: discuss your situation/feelings and ask for what you actually want.
Negotiate: I didn't need to negotiate in the end, but there was some mention of that (e.g., that if this was a problem for her, and she was unable to respect the request, I would know that and act accordingly, possibly by just not going if it didn't feel comfortable for me; that in the future, I would be happy to listen as she directly asked for something she wanted, etc.)
In the end, it all worked out quite well. Our conversation went well. We ordered another drink and walked home together afterward. It was uncomfortable, but I felt heard. My SO and I went camping with my friends, it wasn't the big huge group that it could have been, we had a good time (mostly; we did have some problem areas for sure), I felt more confident in myself for being able to handle the situation as an adult, etc. My SO's OSO did not go, and she and my SO had a little extra time between our camping trip and the OSO's trip to the other place, and if the OSO has any lasting hurt from this, I haven't heard about it and it's her responsibility to manage that and talk about it as needed.
It may have helped that the OSO herself does not seem to be personality disordered in any way, but I felt it was good practice for me regardless and a good way to reduce my anxiety. I essentially framed things the same way when discussing it with my girlfriend, and it helped there, too.
There are numerous conversations we've had go south that when I think back on them, I know DEARMAN would have helped tremendously.
My acronym OSO stands for "other significant other." My uBPDso has another SO besides me. Theirs is a platonic relationship, but it's as involved a "partnership" as mine is with my SO, and the three of us all live together (for the time being; this is changing in the next two weeks -- yay!).
The OSO is very insecure and has a hard time understanding that she is important and valuable to my SO, and that their relationship can be respected as important, without everything being equal all of the time. Me stating that I wanted not to have to worry about time being evenly spent and so on was not something she wanted to hear. But DEARMAN helped.